The present invention relates to a system for realizing a service to reproduce a stored media via a network in real time, such as a VOD (video on demand) service. More specifically, the invention relates to a system for properly using various service classes of a network for each service suitable for its characteristic.
The present invention also relates to a system for realizing a service to offer a media information stored in a server, to a user via a network in real time on his/her demand. In particular, the present invention is directed to a system for data such as managed in time series. Specifically, the invention is directed to a system for data having characteristic wherein the use of the data is managed by a time information such as a time stamp. Throughout this specification, these data will be referred to as real-time data.
In recent years, interactive video services have begun to be made fit for practical use due to the developments of high-speed, wide-band networks and digital image techniques. One of typical examples of such services is a VOD service. The VOD service is a service in which a video server for storing a video source therein is connected to a client basically in the form of one-to-one so that the video source is outputted from the video server generally on the client's demand for reproduction, search, special reproduction or the like. It is assumed that most of the VOD services utilize an ATM (asynchronous transfer mode) for at least the key of a network. Furthermore, throughout this specification, video servers include a temporary memory type server, called a cache node or a network cache, in which a video is stored on or near a communication line set for transmitting the video information which is being offered to a user, and the same video is reused on another user's demand.
In the ATM, a number of classes are provided. For example, in the ATM forum serving as an organization for establishing the trade standard of ATM, the following five classes are standardized. That is, there are CBR (Constant Bit Rate), RT-VBR (Real Time Variable Bit Rate), NRT-VBR (Non-Real Time Variable Bit Rate), ABR (Available Bit Rate) and UBR (Unspecified Bit Rate). Among these classes, the service class of CBR is generally used in the present circumstances since the video transmitting service as set forth above is a real time service.
Then, there will be considered the five service classes as set forth above. From the standpoint of a real-time characteristic, the qualities of two classes including the CBR and the RT-VBR, which are guarantee classes, are guaranteed. In addition, although the CBR has an extremely low probability of discarded cells and a high quality, the cost thereof is high since it does not use the statistical multiplexing effect which is considered to be particularly effective as a method for effectively utilizing bands in the ATM. The RT-VBR is cheaper than the CBR since it uses the statistical multiplexing effect, although the cell discarding characteristic thereof is not better than the CBR. On the other hand, although the real-time characteristics of the NRT-VBR, the ABR and the UBR are not guaranteed, it can be expected that the communication cost is greatly decreased because these services are the best effort services for effectively using space bands.
In the conventional image transmitting systems as set forth above, the transmitting quality is constant in the CBR. For example, consider the situation wherein a two-hour video is watched via the VOD. In general, as the required qualities for the communication call in the VOD, the following matters are guessed: a real-time characteristic for immediate offer, a low rate of discarded cells for ensuring the qualities of image and voice, and a low-cost communication for realizing to decrease the cost of a video service. From the points of the guarantees of the real time characteristic and the cell discarding characteristic, the CBR is selected as a premise.
In general, in the case of the CBR, a band is assigned at a peak cell rate in a call connecting procedure, and this band remains being constantly set while the call is set. The CBR has a good quality since communication is performed after sufficiently ensuring the band. However, since this system does not use the multiplexing effect, the communication cost is high. For example, consider the VOD as set forth above. It is required to set the call while the video is watched. Considering that an image is transmitted to a communication line of 155 Mbps by a MPEG system of 6 Mbps, an image of 25 lines can be simultaneously delivered. On this condition, the communication cost is set. Of course, since the cost for software is also required, it is important to reduce the communication cost. In the conventional systems, there is a problem of decreasing the cost.
In addition, in the conventional system, the video, which has been stored for the previous client, is reused when another client accesses thereto. However, since the storage capacity of the data storage unit is limited, it is required to successively erase videos having a low frequency in access. In this case, the erasing of videos must be performed for each video source. Therefore, in a case where another client accesses immediately after the video source is erased, it is required to perform the transmission via the CBR again, the aforementioned problems become worse.
Since video sources to be offered in VOD services do not include only movies put on the screens in movie theaters, but also include many categories of video sources such as TV movies, dramas and documentary films, and also include all the products which have been produced up to now since a movie has been made, the number thereof is extremely vast. On the other hand, since the memory capacity of a video server for storing such video sources is limited, it is impossible to store all the video sources in a single video server. Also, the quantity of information which is able to be read out of a single video server at the same time is limited. Therefore, in general, it has been considered to adopt a two-hierarchy configuration wherein video servers for storing video sources, which are expected that the frequency in access is high, are arranged at locations near the users every local area, and video servers for storing video sources, which are expected that the frequency in access is low, are arranged at a center remote from the users, or a multi-hierarchy configuration in accordance with the frequency in access.
Thus, in a case where video servers for storing video sources to be offered are dispersively arranged on a network, it is required to establish a communication line between a user and a video server nearest the user among video servers, which have stored video sources on the user's demand, to transmit a video through the established communication line from the standpoint of communication cost. Conventional systems are designed that a user determines a video by a previously distributed video list or the like to directly designate the video. Therefore, after the user designates the video, it is enough to determine the optimum video server for the offer of the video to establish the communication line.
However, it is considered in the future that it will be possible for a user to establish a communication line between the user and the most terminal video server to finally determine a video, that the user wishes to view, while the user interactively accesses a video list displayed on a screen, a part of video sources prepared for preview, and a promotion video consisting of highlight scenes. In such a case, from the standpoints of the responsibility to the user and the communication cost, it is desired to store video lists or video information for preview in the terminal video servers. In this case, if the user actually attempts to establish a communication between the user and a video server having stored a video source, which contains a video that the user wishes to view, after the user determines the video, it is possible to establish the communication line when the communication network is crowded, so that it is not possible to offer the video to the user. In addition, when the number of services offered at the same time by the server for storing the video source reaches the performance limit of the server, it is possible to offer the video even if it is possible to establish a communication line. Thus, if a user attempts to ensure communication network resources and server resources after determining a video that the user wishes to view, the user may waste time for determining the video that the user wishes to view, so that it is extremely inconvenient for the user.
As mentioned above, although it is required to realize a service of a low call-discarding rate in order to realize the immediate offer of image in the conventional image transmitting system, there are problems of decreasing the costs of image transmitting services and of effectively utilizing communication resources.